Adding to the headers included in port-irix.c allows the build
to complete.

Using c99, there are only 92 warnings, whereas using cc there are 251.
A reasonable number of the 92 warnings are due to macro redefinition:

14 Macro "_PATH_MAILDIR" (declared at line 359 of "defines.h") has an
14 Macro "_PATH_BSHELL" (declared at line 322 of "defines.h") has an
incompatible
5 Macro "offsetof" (declared at line 491 of "defines.h") has an
incompatible
1 Macro "offsetof" (declared at line 491 of "../defines.h") has an
incompatible

I haven't had time to deal with the regression tests yet. The protocol 1
tests always seem to be a problem with IRIX. (I reported this once quite
a while ago, but did not have the time to investigate.) In the past,
I've hacked the scripts to only test protocol 2, in which case
everything (that we care about) is fine.